I went there with my family two days ago.
My younger sister specializes in ticketing, so she got me a same-day cancellation ticket.
Gyeongbokgung Palace Night Opening Tips) Tickets are issued starting around 5:40, admission is at 6:00
As soon as you enter, you can run to Gyeonghuiru, sit on a bench, wait until sunset, set up a tripod, and take a long exposure.
Tripods are not prohibited, but staff will stop you if you set up a tripod over or over the no-entry fence.
There were a lot of ladies who came in groups with tripods attached to their cameras, but the staff kept stopping them because of this.